The present invention relates to the field of color image reproduction and, more particularly, is directed to a method and apparatus which greatly improves the quality of a reproduced color image.
Advancements in color image technology in recent years has been accompanied by considerable research and development directed to practical applications of color photocopiers and printers. There are two types of color photocopiers presently known in the art. In one type, color reproduction is accomplished by photoelectric conversion of the original color image to electronic signals which are processed and used to form the reproduced color image. This method, however, is deficient because of poor resolution of the reproduced image and lack of quality due to the generation of noise components during the photoelectric conversion process. In another type of photocopier known in the art, the color image is formed by chemical processing in the same manner as in conventional photocopiers. Though the electronic conversion method of color image reproduction has the above mentioned deficiencies, it has received a great deal of attention because it does not require the cumbersome handling of chemicals and can be used to process images in a variety of ways.
In a conventional color image reproduction system where the original image is electronically processed, the original image is electronically scanned through color filters to provide three color signals. These analog signals are then converted to digital signals by an analog to digital convertor (A/D convertor). The digital signals are then subjected to separation and conversion of a luminance signal and two subtracted color signals. The luminance and subtracted color signals are then converted to output signals by a ROM based conversion table for application to a reproduction device such as a photocopier or printer.
The conversion table converts the luminance and subtracted color signals to ink concentration signals which match the characteristics of the inks used by the particular photocopier or printer. Processing the scanned image signals by an A/D convertor and performance of the other processing steps, such as shading correction, gamma adjustment and separation into luminance and subtracted signals, can be done by comparatively simple circuitry. However, the conversion of the luminance and subtracted color signals to the requisite ink concentrations is generally a very complex operation. Thus, a look-up table is usually used for the conversion. Mathematical calculations are used to develop the conversion table so that specific combinations of ink concentrations for reproducing colors can be determined for the particular sets of luminance and subtracted color signals. Such mathematical calculations are disclosed by John A. C. Yule in a publication entitled "Principle of Color Reproduction," 1967, pp. 255-281. In actual practice, however, not all the specific sets of luminance and subtracted color signals can be converted to corresponding ink concentrations which can be reproduced by the reproduction device. Thus, no information is reproduced for these signals. In practice, there is a strong likelihood that luminance and subtracted color signals which have no corresponding ink concentrations will be produced due to the generation of random noise components in the system. Thus, the quality of the reproduced image will be greatly deteriorated.